sched: Clarify init and policy selection routine

`sched_select()` explicitly work as a scheduler initialization routine.
It does the following tasks 1) select scheduler policy 2) initialize
relative scheduler structure for specified policy. However, we seperate
it into two funcs. The one is `sched_init()` and the other one is still
`sched_select`.

It would also meet the expected behavior within `start_kernel`. We expect
a function to initialize the scheduler relative structure and then select
the specified policy via `sched_select()`. `start_kernel()`.

closed #43

include/kernel/sched.h

@@ -24,6 +24,7 @@ struct sched {
     int (*elect)(int switch_type);
 };
 
+int sched_init();
 int sched_select(int sched_type);
 int sched_enqueue(struct thread_info *thread);
 int sched_dequeue(struct thread_info *thread);

kernel/main.c

@@ -130,7 +130,9 @@ struct thread_info *start_kernel(void)
     show_page_bitmap();  // init_pages();
     kmem_cache_init();
 
-    /* select a scheduling policy */
+    /* initialize the scheduler internels */
+    sched_init();
+    /* select giving scheduling policy */
     sched_select(SCHED_CLASS_BITMAP);
 
     /* idle_thread is not added to the runqueue */

kernel/sched.c

@@ -6,6 +6,16 @@ extern const struct sched sched_bitmap;
 
 static const struct sched *sched;
 
+int sched_init()
+{
+    int ret;
+
+    ret = sched_rr.init();
+    ret = sched_bitmap.init();
+
+    return ret;
+}
+
 int sched_select(int sched_type)
 {
     switch (sched_type) {
@@ -19,7 +29,7 @@ int sched_select(int sched_type)
         return -1;
     }
 
-    return sched->init();
+    return 0;
 }
 
 int sched_enqueue(struct thread_info *thread)